This response is based on the information provided.  We suggest you discuss with your contracting team, program manager and/or legal department as appropriate. 

A J&A will be needed to make this modification as you noted.  Whether to create a new CLIN or modify the existing CLIN is a matter of choice.  The most important thing to consider is how the modification affects payment.  Your instructions about payment need to be clear.  It may save you time later to speak with DFAS to see how they will process payment based on the way use modify the contract.  You can add a new CLIN and that may be the cleanest method for payment purposes.  This indicates the new work is discreet from the work already on CLIN.